I think that's unlikely because M-Harry doesn't exist until the end of Grave Peril. Additionally, it probably takes M-Harry a while to become evil.
What if M-Harry in Harry's mind is from some point in the future? We can assume that Harry's universe and the Mirror-universe are concurrent, time-wise. But what is it that CAUSES Harry to make the different decision that creates the split? Do we just assume the Many-Worlds Theory that everything that can happen does? Or did something trigger it?
There's no reason to assume that if M-Harry is a warlock and (in my theory) using black magic to invade Harry's mind, that he wouldn't also go back in time. In for a penny, in for a pound.
M-Harry sends his mind back in time to his earlier self. Puts thoughts into his own head to change something that M-Harry did (not go to Bianca's party, kill Susan in the basement, etc...). This CREATES the divergent timeline. M-Harry uses his connection to our Harry to swap and escape his own.
